1. Field of the Invention
The present invention relates to a loudspeaker device, and more particularly to a loudspeaker device which implements satisfactory bass reproduction using a compact cabinet.
2. Description of the Background Art
As the digitization of audio equipment proceeds, it has become possible to readily reproduce a bass signal contained in a music source using a compact apparatus, e.g., a portable CD player, or a DVD player. However, a loudspeaker device, which performs final sound reproduction, requires a large cabinet volume for bass reproduction, and therefore there is a difficulty in realizing a compact loudspeaker device capable of satisfactory bass reproduction.
Various systems have been proposed for realizing a loudspeaker device capable of satisfactory bass reproduction using a compact cabinet. For example, in a bass-reflex system which is currently the most popular bass reproduction system, an acoustic port is provided to a cabinet in which a speaker unit is provided, and an acoustic resonance determined by an acoustic stiffness, which is caused according to a cabinet volume, and acoustic mass of the acoustic port is used for extending a bass reproduction range. However, even in the bass-reflex system, there is a difficulty in realizing satisfactory bass reproduction unless the acoustic stiffness caused according to the cabinet volume is decreased, i.e., unless the cabinet volume is increased.
For example, Japanese Patent Laid-Open Publication No. 2000-308174 discloses a conventional loudspeaker device having an improved bass reproduction limit which is determined based on a cabinet volume. The conventional loudspeaker device disclosed in Japanese Patent Laid-Open Publication No. 2000-308174 is described below with reference to FIG. 11. FIG. 11 is a cross-sectional structure diagram of the conventional loudspeaker device.
In FIG. 11, the conventional loudspeaker device is generally structured by a speaker cabinet 1 and a speaker unit 2. The speaker unit 2 includes a center pole 3, a magnet 4, a plate 5, a voice coil 6, a voice coil bobbin 7, a frame 8, a damper 9, a cone diaphragm 10, an edge portion 11, a dust cap 12, a movable magnet 13, and a fixed magnet 14. The plate 5 has its surface fixed on an upper face of the magnet 4 (i.e., a surface of the magnet 4 which faces the diaphragm 10). The voice coil 6 wraps around an outer surface of the voice coil bobbin 7, and is situated in a magnetic gap between an outer surface of the center pole 3 and an inner surface of the plate 5. The frame 8 is fixed on an upper face of the plate 5 (i.e., a surface of the plate 5 which faces the diaphragm 10). The damper 9 has its outer edge fixed on the frame 8, and supports the outer surface of the voice coil bobbin 7. The cone diaphragm 10 is fixed around an upper end portion of the voice coil bobbin 7. The edge portion 11 is situated between the frame 8 and the diaphragm 10 so as to support an outer edge of the diaphragm 10. The dust cap 12 is fixed on the diaphragm 10. The movable magnet 13 has a ring-like shape, and its inner surface is fixed on the outer surface of the voice coil bobbin 7. The fixed magnet 14 has a ring-like shape, and its inner surface is opposed to the outer surface of the movable magnet 13 so as to form a gap between the fixed magnet 14 and the movable magnet 13. The fixed magnet 14 is magnetized so as to have the same polarity as that of the movable magnet 13 in a thickness direction.
Described next is an operation of the thus-configured conventional loudspeaker device. When an electric signal is applied to the voice coil 6, a drive force is generated. As in an ordinary loudspeaker, the drive force vibrates the cone diaphragm 10 connected to the voice coil bobbin 7, thereby generating sound. This conventional loudspeaker is considerably different from ordinary loudspeakers due to an interaction between the movable magnet 13 fixed on the outer surface of the voice coil bobbin 7 and the fixed magnet 14 opposed to the movable magnet 13. When the cone diaphragm 10 is vibrated by the drive force generated in the voice coil 6, the movable magnet 13 attached to the voice coil bobbin 7 is also caused to vibrate inside an inner edge of the fixed magnet 14. As described above, the movable magnet 13 and the fixed magnet 14 are magnetized so as to have the same polarity as each other in the thickness direction, and therefore the movable magnet 13 and the fixed magnet 14 repel each other. Accordingly, if the movable magnet 13 deviates from the center of the fixed magnet 14, so that they are no longer magnetically balanced with each other, the movable magnet 13 acts to apply a force away from the center of the fixed magnet 14, i.e., a negative stiffness, to a vibration system of the speaker unit 2. Due to a magnetic force generated in the movable magnet 13, a force functioning as the negative stiffness reduces a bouncing force of an acoustic stiffness of the cabinet 1. As a result, the loudspeaker device having a small cabinet is able to realize satisfactory bass reproduction as if the loudspeaker unit is provided in a larger cabinet.
In the conventional loudspeaker device disclosed in Japanese Patent Laid-Open Publication No. 2000-308174, however, a negative stiffness generation mechanism, i.e., the movable magnet 13 and the fixed magnet 14 are provided in the speaker unit 2, and therefore the structure of the speaker unit 2 is complicated. Moreover, since the movable magnet 13 is attached to the voice coil bobbin 7, the weight of the vibration system is increased, resulting in a reduction of an output sound pressure level of the speaker unit 2.
Further, the negative stiffness is set so as to reduce the acoustic stiffness caused in accordance with the cabinet volume. Accordingly, in the above conventional loudspeaker device, the cabinet 1 is required to be sealed so as not to cause air to leak therefrom, and therefore there is a difficulty in employing a bass-reflex cabinet which is advantageous in extending the reproduction limit to a frequency band lower than that of the reproduction limit for the sealed cabinet.